1. Field of the Invention
The present invention relates to an image adjustment method and an image forming apparatus of electrophotographic type, and more specifically relates to an image adjustment method and an image forming apparatus capable of automatically adjusting misregistration of a multi-color image, which is caused when forming the multi-color image by superimposing a plurality of color component images on a recording carrier.
2. Description of Related Art
In an image forming apparatus such as a digital color copying machine and a digital color printer, after decomposing inputted data into respective color components and performing image processing, images of the respective color components are superimposed to form a multi-color image. If the respective color component images are not accurately superimposed during the formation of a multi-color image, misregistration occurs in the resultant multi-color image, and image quality deteriorates. In particular, in an image forming apparatus comprising an image forming section for each color component so as to improve the formation speed of a multi-color image, the multi-color image is formed by forming respective color component images in the respective image forming sections and superimposing the respective color component images one upon another. In such an image forming apparatus, there tend to be differences among the transfer positions of the respective color component images, and consequently there arises a serious problem of misregistration of the multi-color image.
Therefore, in order to accurately superimpose the respective color component images, the image forming apparatus performs color registration adjustment for correcting the misregistration of a multi-color image, so that a satisfactory multi-color image having no misregistration is formed. The color registration adjustment is usually carried out by using an optical detector to detect the displacement of the image forming positions of other color components with respect to the image forming position of a color component to be the reference. Next, a correction amount is determined based on the result of the detection, and then, according to the correction amount, the timing of forming respective color component images is adjusted so that the transfer positions of the respective color component images agree with each other. In general, in order to determine a correction amount, the respective color component images are transferred at the same timing and the distance between the transfer positions of the respective color components is detected, or the density of a multi-color image formed by superimposing the respective color components is measured.
For example, in an image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 10-213940 (1998), the distance between the transfer positions of the respective color component images is detected, and a correction is made based on the detected amount of displacement between the transfer positions. In this image forming apparatus, the distance between an image formed by a color component to be the reference and images formed by other color components is detected with a detector, the amount of displacement between the transfer positions of the respective color component images is determined based on the detected distance, and the misregistration is corrected.
Further, Japanese Patent Application Laid-Open No. 2000-81744 discloses an image forming apparatus which measures the density of a multi-color image formed by superimposing respective color component images, and corrects misregistration so that the measured density becomes equal to a density which is obtained when the respective color component images are accurately superimposed. In this image forming apparatus, in order to improve the correction accuracy, a plurality of same images of each color component are repeatedly formed. According to the above publication, a plurality of line images are formed as the same images, and the density of a multi-color line image is detected with a detector so as to find the superimposed state of the respective color component line images. Then, a state in which the density of the multi-color line image detected with the detector is within a predetermined density range is regarded as a state in which the respective color component line images are accurately superimposed, and a correction is made so that image formation is performed in this superimposed state, thereby performing the color registration adjustment.
Thus, when performing the color registration adjustment by measuring the position or the density of the formed image and detecting the positional relation of a color image subjected to correction with respect to a color image to be the reference, the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 10-213940 (1998) does not need to form a large number of line images since it detects the position of the line images. Whereas, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-81744 in which a plurality of line images are formed and the density of a multi-color line image is detected with a detector so as to find the superimposed state of the respective color component line images, the number of respective color component line images to be formed is influenced by the sampling cycle of the detector. If the sampling cycle is short, the number of line images to be formed can be reduced. Whereas, if the sampling cycle is long, the number of line images to be formed must be increased. The sampling cycle of the detector is set based on an operation clock of control means. Usually, this control means always monitors the input and output of detecting means or the like which is installed in the image forming apparatus to know the condition of the apparatus, and is in a standby state for signals from various kinds of detecting means or the like so that it can respond quickly to instructions from the outside. Therefore, the sampling cycle for detecting line images cannot be set short when executing the color registration adjustment, and consequently the number of line images to be formed increases. In addition, there is a problem that the adjustment time is longer. Further, in order to avoid taking a long time to make the adjustment, the number of samplings needs to be decreased, and thus there is a problem that highly accurate detection cannot be performed.